Conducting polymer with actively switchable absorbency

ABSTRACT

Structure with electrically switchable wettability. The structure includes a doped conducting polymer, a counter electrode and an electrolyte disposed between the doped conducting polymer and the counter electrode. A preferred conducting polymer is polypyrrole doped with fluorinated carbon ions. A voltage between the doped conducting polymer and the counter electrode will cause the conductive polymer to switch between hydrophobic and hydrophilic states.

This invention was made with Government support under Contract No. W911NF-07-D-0004, awarded by the Army Research Office. The Government hascertain rights in this invention.

BACKGROUND OF THE INVENTION

The present invention relates to structure with switchable wettabilityand more particularly to such a structure that can be reversiblyswitched electrically between hydrophobic and hydrophilic states.

Materials are known whose wettability may be switched. These materialsmay be thermally responsive, pH responsive, and photo responsive ineffecting the switch from a hydrophobic to a hydrophilic state. Anothermethod of switching wettability is electrowetting, where an electricfield applied between the material and the fluid changes the surfacetension of the fluid and causes it to wet the surface. The wetting stateof a surface is quantified by the contact angle that a water dropletmakes with the surface—a hydrophobic surface has a contact angle greaterthan 90° and a hydrophilic surface has a contact angle less than 90°.Special non-wetting, or superhydrophobic surfaces are characterized bycontact angles greater than 150°, while fully-wetting, orsuperhydrophilic, surfaces are characterized by contact angles less than5°.

Conducting polymers offer advantages over other methods of switchingwettability because of their fast electrochemical switching, lowoperating voltage and ease of fabrication. Polypyrrole is a conductingpolymer. Previously, polypyrrole required immersion in an electrolyte inorder to switch its wettability between hydrophobic to hydrophilicstates by electrically inducing a change in the chemical composition ofthe surface. This severely limited the applications for whichpolypyrrole could be used. A research group has, however, switched thewettability of polypyrrole without immersion in electrolyte by creatinga mechanical change in the surface features. This mechanical surfacemodification was able only to change the degree of hydrophobicity of thematerial. The material switched from superhydrophobic (contactangle=152°) to “medium hydrophobic” (contact angle=131°) rather thanfrom a superhydrophobic (contact angle=164°) to superhydrophilic state(contact angle=0°) as in the present invention. See, Chen, T.-H., etal., “A Wettability Switchable Surface by Microscale Surface MorphologyChanges,” Journal of Micromechanics and Microengineering, 17 (2007):489-495.

Another conducting polymer is polyaniline. A research group hasdemonstrated a wettability switch with polyaniline without immersion inelectrolyte but this group was able only to switch the surface between aslightly hydrophilic state (contact angle=37°) and a more hydrophilicstate (contact angle=9°). They could not switch between superhydrophobicand superhydrophilic states. See, Isaksson, J., et al., “A Solid-StateOrganic Electronic Wettability Switch,” Advanced Materials, 16.4 (2004):316-320.

An object of the invention, therefore, is to create a structure thatallows switching of the conducting polymer such as polypyrrole fromhydrophobic to hydrophilic without full immersion in an electrolyte.

SUMMARY OF THE INVENTION

In one aspect, the structure according to the invention has electricallyswitchable wettability. The structure includes a doped conductingpolymer, a counter electrode and an electrolyte disposed between thedoped conducting polymer and the counter electrode. A preferredconducting polymer is polypyrrole. Another suitable conducting polymeris polyaniline. In a preferred embodiment, the polypyrrole is doped withfluorinated carbon ions such as perfluorooctanesulfonate andnonafluorooctancesulfonate ions. In applications where biocompatibilityis important, the polypyrrole polymer may be doped with biocompatiblechemicals such as sodium dodecylbenzenesulfonate (NaDBS). In thispreferred embodiment a suitable counter electrode is gold foil. Thestructure may include a moisture sensor to trigger the switch fromhydrophobic to hydrophilic or vice versa. A means is provided forestablishing a voltage between the conducting polymer and the counterelectrode.

The structure disclosed herein may be incorporated into consumerproducts such as diapers and feminine hygiene products. The structuremay also be incorporated into micro-fluidics systems. Further, thestructure may be coated upon a surface.

In another aspect, the structure according to the invention incorporatesan electrically switchable wettability gradient portion and includes adoped conducting polymer along with a counter electrode. An electrolyteis disposed between a portion of the doped conducting polymer and thecounter electrode so that a portion of the structure may be switchedfrom a hydrophobic to hydrophilic state and back again. A wettabilitygradient can also be created by applying different voltages toelectrically isolated portions of the polymer. This structure can beused to move a liquid droplet or to control a liquid droplet contactangle so as to make a variable refractive power lens.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of one embodiment of the presentinvention.

FIG. 2 is a photograph of a prototype created for experimental purposes.

FIGS. 3 a and 3 b are schematic illustrations of an embodiment of theinvention in which the electrolyte region covers only a portion of theconductive polymer.

FIG. 4 is a photomicrograph of a thin film of polypyrrole.

FIG. 5 are photographs of droplets placed on polypyrrole in differentdoped states, showing that the surface can be switched betweensuperhydrophobic and superhydrophilic states by reduction and oxidation.

FIG. 6 is a graph of sodium chloride concentration in water versuscontact angle.

FIGS. 7 a-7 d are photographs of a droplet moving across a wettabilitygradient.

FIGS. 8 a-8 d illustrate a droplet being absorbed into the structurewhen the wettability is switched to hydrophilic.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference first to FIG. 1, a switchable device or structure 10includes a polypyrrole layer 12, an electrolyte layer 14 and a gold foilcounter electrode 16. The counter electrode 16 need not be gold foil. Itcan be any chemically inert conductive material (e.g., stainless steel).It can also be a conducting polymer such as polypyrrole. A voltagesource 18 provides an electrical potential between the polypyrrole film12 and counter electrode 16. The polypyrrole film 12 has the ability tobe activated with a low voltage which may be less than 5 volts. Thepolypyrrole film 12 is doped with a fluorinated carbon such asperfluorooctanesulfonate or nonafluorooctanesulfonate ions which move inand out of the material with an electrical stimulus. Another fluorinatedcarbon ion that may be used is perfluorobutanesulfonate. The fluorinatedcarbon can also be a fluorosurfactant. The ionic motion changes thechemical composition and therefore the hydrophobicity of the material.An advantage of using fluorinated carbon materials as the dopant is thewide range of hydrophobic/hydrophilic conditions that are possible. Thematerial can switch from a completely non-wetting (superhydrophobic) toa fully wetting (superhydrophilic) state. When the material is fullydoped, it is in the completely non-wetting state, and when it is undoped(concentration of dopant ions close to zero), the material is in thefully wetting state. The material can be partially doped to be in anintermediate, partially wetting state. The fully-doped material containsthe maximum concentration of dopant that can be driven into the polymer.

In applications where biocompatibility is a concern (i.e., diapers,feminine hygiene products), the polymer can be doped with chemicalsknown to be biocompatible such as sodium dodecylbenzenesulfonate(NaDBS), and still exhibit a hydrophobic-hydrophilic switch. Those ofordinary skill in the art will recognize that a moisture sensor can beprovided to cause the structure 10 to switch its state when moisture isdetected. In this case, the polymer can switch into a storage mode toprevent moisture from exiting the material. In the storage mode, thepolymer is completely absorbent and will absorb any moisture thatcontacts the polymer. The polymer can also be switched to anintermediate absorbency state, where the polymer is switched to apartially wetting state. In the intermediate absorbency state, thepolymer will be able to absorb a fraction of the moisture that comesinto contact with the polymer.

The electrolyte layer 14 is ionically conductive but electricallyinsulating. Applying a voltage between the two layers 12 and 16 willdrive ions that make the material hydrophobic out of the polypyrrole,through the electrolyte 14 layer and toward the counter electrode 16resulting in a hydrophilic surface. By reversing the voltage the ionscan be driven back into the polymer resulting once again in ahydrophobic surface.

FIG. 2 is a photograph of a prototype created for experimental purposes.This prototype in FIG. 2 has demonstrated a switch from asuperhydrophobic to a superhydrophilic state. The transition time can beas fast as 20 milliseconds. The transition time can be adjusted by thetype of electrolyte used. Suitable electrolytes and their transitiontimes are shown in Table 1.

TABLE 1 Transition times for various electrolytes. ElectrolyteTransition time 0.1M potassium perfluorooctanesulfonate + 1-3 secpropylene carbonate + 25 wt % polymethylmethacrylate 0.03M potassiumperfluorooctanesulfonate 30-120 sec in cellulose-based gel 0.015Mpotassium perfluorooctanesulfonate 20 msec in deionized water

In general, the first electrolyte in Table 1 is a salt (such aspotassium perfluorooctanesulfonate) dissolved in an organic solvent(such as propylene carbonate, ethylene carbonate, acetonitrile, or acombination of these solvents) with poly(methyl) methacrylate (PMMA)added as a stiffener. The second electrolyte in Table 1 is a saltdissolved into a cellulose-based gel, and the third electrolyte in Table1 is a salt dissolved in deionized water.

With reference now to FIGS. 3 a and 3 b, the electrolyte region 14extends below only a portion of the polypyrrole film 12. Thisarrangement allows a wettability gradient to be created on the surfaceby controlling which regions of the film are in contact with theelectrolyte. As shown in FIG. 3 a, a liquid droplet 20 may be caused tomove toward the right (FIG. 3 b) when the region above the electrolytebecomes hydrophilic. Thus the present invention can be used to movedroplets or to cause droplets to interact with one another to improvemixing. A wettability 170 gradient can also be created by activatingelectrically isolated regions of the polymer, therefore switching onlycertain regions of the polymer.

With reference to FIG. 4, the polymer film 12 exists as a thin film withrough microstructures on its surface. The rough microstructures act toamplify the inherent hydrophobicity or hydrophilicity of the material.By varying the deposition conditions, the film thickness can beadjusted. Table 2 shows that film thickness can be adjusted by varyingthe current density during which the film is grown. The microstructurescan be added to the material with either soft-template or hard-templatemethods.

TABLE 2 The film thickness can be adjusted by varying the currentdensity that the film is grown at. Current Underlying FilmMicro-structures Density(A/m²) Thickness (nm) height (μm) 0.5 50 10-251.0 70 10-25 1.5 178 15-30 2.0 180 25-40 2.5 160 35-40

The polymer can be switched between superhydrophobic (contact anglegreater than)150° and superhydrophilic (0° contact angle) states. FIG. 5illustrates that the surface can be switched from superhydrophobic tosuperhydrophilic states by reduction and oxidation.

The structure 10 disclosed herein can be used to repel and absorb avariety of fluids including water. The polymer has demonstratedsuperhydrophobicity with other high surface tension fluids, such asglycerol and saltwater (aqueous NaCl solution). The polymer disclosedherein has demonstrated a high hydrophobicity to salt water withsalinity comparable to seawater (0.6 M NaCl in H₂O) with a contact angleof 142°. FIG. 6 is a graph showing the effect of NaCl concentration oncontact angle. Table 3 shows contact angles of various fluids on thepolymer.

TABLE 3 Contact angles of various fluids on the polymer. SurfaceTension, Contact Angle Name γ_(iv), at 20° C. (mN/m) (deg) Water 72.8164 Glycerol 64.0 146 Ethylene glycol 47.7 51 Propylene glycol 38.0 0Oil 20-25 0 6.0M NaCl in water  82.55 76

The switchable structure disclosed herein retains its switchability forat least 100 cycles with only slight degradation occurring. Table 4illustrates that the polymer can last at least 100 cycles.

TABLE 4 The polymer can last at least 100 cycles. Cycle Number θ*oxidized θ* reduced 0 (initial) 161 0 10 154 0 50 155 0 100 148 0

Experiments have been conducted to establish the efficacy of thestructure disclosed herein. FIGS. 7 a-7 d illustrates the movement of adroplet of liquid toward the right as the wettability is switched. FIGS.8 a-8 d show a liquid droplet being absorbed when the structure isswitched from hydrophobic to hydrophilic.

Those of ordinary skill in the art will recognize that the structuredisclosed herein will 215 have myriad applications. In addition todiapers and feminine hygiene products, Mops and sponges could benefitfrom the structure of the invention. In the diaper context, anabsorbency gradient can be employed so that certain areas of the productcan be more absorbent than others. For example, a diaper with asuperabsorbent bottom and a dry waistline may be desired to containmoisture and prevent leaks. There can thus be zones of absorbency andnon-absorbency 220 as needed and these zones can have the ability tochange location due to the switchable hydrophobicity of the structuredisclosed herein.

Other applications of the structure disclosed herein can be used in thefield of microfluidics such as lab-on-a-chip devices. The structuredisclosed herein can be used to drive fluid droplets across a surfacewithout the need for channels. This functionality can result in lesscontamination and more flexibility in fluid delivery since the fluid canbe driven to any point on the surface and not limited by paths set bychannels. This functionality has many applications including drug,fluid, and nutrient delivery, cell and tissue culture platforms, andhigh-throughput assays.

The structure of the invention can also be used in a micro-mixingcontext. The device can be used to drive two micro-droplets together topromote mixing. The active mixing process is much faster than thepassive diffusion process. The present invention can also be used forcoatings for microfluidic channels to allow researchers to control thefluid flow inside a channel. Fluid flow can be stopped and started oncommand.

The present invention can also be used in smart cooling systems in whichwater droplets are delivered to hot spots that can develop on heatexchangers and electrical circuits. The structure disclosed hereinprovides a faster, more efficient method of cooling. The invention canalso be used for water harvesting. Water droplets can collect and growon hydrophilic regions of the structure. Once the droplets have grown toan appropriate size they can be released and collected as the surface isswitched to hydrophobic.

The present invention may also be used to make liquid lenses havinginfinitely-variable refractive index by controlling the liquid contactangle and without the need for any moving parts. The invention can alsobe used to form self-cleaning surfaces. The polymer can be used as acoating on any surface. When water droplets contact the coating, theywill roll off when the polymer is tilted at a low angle (10°), andcollect any dirt or particles that have collected on a surface.

Another important use for the structure disclosed herein is for lowfriction surfaces or surfaces with variable friction characteristics.For example, the polymer can be used as a coating on pipes, submarines,boats, or any other water vehicle. In the superhydrophobic, non-wettingstate, the friction coefficient between water and the polymer isextremely low. In the superhydrophilic wetting state, the frictioncoefficient between water and the polymer is high. The polymer can beswitched to the non-absorbent state thereby allowing the vehicle to movevery fast since drag has been reduced. When the polymer is switched tothe absorbent state, drag is increased and the vehicle can slow downmuch faster than without the coating. It will also be apparent thatweatherproof materials can utilize the polymer coated on clothing orother products such as camping gear, umbrellas, and boots that need tobe waterproof.

The polymer disclosed herein exhibits oleophilic (oil-absorbent)properties. In the non-wetting state, the polymer can absorb oil andrepel water which can be used for oil and water separation or filteringapplications. The polymer can absorb both oil and water when switched tothe wetting state.

Heretofore, a significant disadvantage of the polypyrrole conductingpolymer was that it required immersion in an electrolyte in order for itto be switched. The inventors herein are aware of no group that has ofyet been able to switch the wettability of polypyrrole without immersionin an electrolyte and without a mechanical change in surface features.The present invention thus makes conducting polymers a viable candidateas a material to be used in the above-mentioned applications.

Modifications and variations of the invention disclosed herein will beapparent to those of ordinary skill in the art. It is intended that allsuch modifications and variations be included within the scope of theappended claims.

1. Structure with electrically switchable wettability comprising: adoped conducting polymer; a counter electrode; and an electrolytedisposed between the doped conducting polymer and the counter electrode.2. The structure of claim 1 wherein the conductive polymer ispolypyrrole.
 3. The structure of claim 1 wherein the conductive polymeris doped with fluorinated carbon ions.
 4. The structure of claim 3wherein the fluorinated carbon ions are perfluorooctanesulfonate ornonafluorobutanesulfonate.
 5. The structure of claim 1 where in theconducting polymer is doped with NaDBS.
 6. The structure of claim 1where in the counter electrode is gold foil.
 7. The structure of claim 1further including a moisture sensor to switch the wettability state. 8.The structure of claim 1 further including means for establishing avoltage between the conducting polymer and the counter electrode.
 9. Thestructure of claim 1 wherein the electrolyte is a salt dissolved in anorganic solvent further including addition of a stiffener.
 10. Thestructure of claim 1 wherein the electrolyte is a salt dissolved in agel.
 11. The structure of claim 1 wherein the electrolyte is a saltdissolved in deionized water.
 12. The structure of claim 1 wherein theelectrolyte is potassium perfluorooctanesulfonate, propylene carbonate,and polymethylmethacrylate.
 13. The structure of the claim 1 wherein theelectrolyte is potassium perfluorooctanesulfonate in cellulose-basedgel.
 14. The structure of claim 1, wherein the electrolyte is potassiumperfluorooctanesulfonate in deionized water.
 15. Structure with anelectrically switchable wettability gradient comprising: a dopedconducting polymer; a counter electrode; and an electrolyte disposedbetween a portion of the doped conducting polymer and the counterelectrode.
 16. The structure of claim 1 incorporated into a diaper. 17.The structure of claim 1 incorporated into a feminine hygiene product.18. The structure of claim 1 incorporated into a microfluidics system.19. The structure of claim 1 coated upon a surface.
 20. The structure ofclaim 1 used to control a liquid droplet contact angle to make avariable refractive power lens.
 21. The structure of claim 1 used toseparate oil from water.
 22. The structure of claim 19 wherein thesurface is a pipe, boat, submarine, or other water vehicle.
 23. Thestructure of claim 19 wherein the surface is clothing.
 24. The structureof claim 15 wherein the structure is used to move a droplet.